We should have a tool that finds unused diagnostic IDs

[beccadax]

While working on #67075, I found several stale diagnostics that are declared in Diagnostics*.def files, but are never used anywhere in the compiler. I deleted most of the ones I found, but I’m sure there are many more.

We should have a tool that scans lib/ and include/ for uses of diagnostic IDs and flags any that are never actually used. This could perhaps be a test that would fail if there are unused diagnostics.

This might be a good first issue for a new contributor who has reasonably strong Python or Swift skills but doesn’t have any experience with compilers yet.

[eddiesze]

Can I take this issue? This is my first time contributing to open source project.

[AnthonyLatsis]

Sure, go ahead.

[eddiesze]

Just to clarify, am I supposed to build diagnostic tools that finds unused diagnostic ID? In addition, Any suggestion on where should I start?

[xedin]

You'd need to load include/swift/AST/DiagnosticsAll.def and for each diagnostic ID in there check whether diag::<diagnostic-id> appears anywhere in lib/ or include/ excluding *.def files.

[XaurDesu]

This could perhaps be a test that would fail if there are unused diagnostics.

Would https://github.com/apple/swift/tree/main/test/diagnostics be a good location for it? I was thinking we could probably add it as a new file, given how generally files on this folder seem to be designed to just check for a specific problem and i'd imagine this would probably fit under that description more or less. Something along the lines of no-unused-diagnostic-ids.swift.